The present invention generally relates to fuel cells and, more particularly, to an improved bipolar plate and method of making the same that is corrosion resistant, has good electrical conductivity, and is low in manufacturing cost.
A fuel cell is a galvanic conversion device that electrochemically reacts a fuel with an oxidant within catalytic confines to generate a direct current. A fuel cell includes a cathode electrode that defines a passageway for the oxidant and an anode electrode that defines a passageway for the fuel. A solid electrolyte is sandwiched between and separates the cathode from the anode. An individual electrochemical cell usually generates a relatively small voltage. Thus, to achieve higher voltages that are useful, the individual electrochemical cells are connected together in series to form a stack. Electrical connection between cells is achieved by the use of an electrical interconnect between the cathode and anode of adjacent cells. Also typically included in the stack are ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
The fuel and oxidant fluids are usually gases and are continuously passed through separate passageways. Electrochemical conversion occurs at or near the three-phase boundary of the gas, the electrodes (cathode and anode) and electrolyte. The fuel is electrochemically reacted with the oxidant to produce a DC electrical output. The anode or fuel electrode enhances the rate at which electrochemical reactions occur on the fuel side. The cathode or oxidant electrode functions similarly on the oxidant side.
One type of fuel cell is a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) cell. In such a fuel cell, a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) is located between two electrodes (cathode electrode and anode electrode) to form a sandwich-like assembly, which is often referred to as a xe2x80x9cmembrane-electrode-assembly.xe2x80x9d The two electrodes are each comprised of a thin sheet of porous material that is permeable to liquid and gas. The two electrodes are situated on either side of a proton exchange membrane such that one surface of each electrode abuts a catalyst layer.
The remaining surface of each electrode respectively abuts a nonporous, gas impermeable, electrically conductive plate. The electrically conductive plate has channels or flow fields for gas flow, and serves as a manifold to distribute fuel gas across the abutting electrode. The two electrically conductive plates are electrically connected together by an external circuit.
Hydrogen fuel gas flows through the grooves in the electrically conductive plate on the anode electrode side, diffuses through the anode electrode, and reacts with the catalyst to produce free electrons and H+ ions. The electrons flow to the cathode electrode by means of the external circuit, and the H+ ions migrate through the PEM to the cathode electrode. Oxygen gas flows through the grooves of the electrically conductive plate on the cathode electrode side and reacts with the H+ ions and free electrons to form liquid water.
In a fuel cell stack, the electrically conductive plates are often referred to as bipolar plates because one face contacts the cathode electrode while the opposite face contacts the anode electrode. Each bipolar plate therefore conducts electrical current from the anode of one cell to the cathode of the adjacent cell in the stack. The electrical current is collected by the two plates at the ends of a stack, known as end plates or current collectors. A stack-design dependent number of thicker plates comprising channels in the plate thickness for a coolant fluid are used to control the temperature of the stack to about 85 C.
However, in the PEM fuel cell environment, the bipolar plates are subject to corrosion due to gases formed by reaction and water. Therefore, in addition to having sufficient electrical conductivity, the bipolar plates have to be corrosion resistant so as to maintain adequate conductivity and maintain dimensional stability over the operational life of the fuel cell.
Graphite bipolar plates have exhibited qualities of sufficient conductivity and corrosion resistance. Yet, graphite plates typically require several manufacturing steps, such as densification with a phenolic resin, followed by high temperature carbonization. Further, graphite bipolar plates, in general, are relatively brittle, particularly when formed as thin sheets in a fuel cell stack, and expensive machining is necessary to form the flow-fields, since near-net shape fabrication of the plates is not possible with graphite.
An example of the use of graphite is shown in U.S. Pat. No. 4,124,747. Therein, it was noted that past compositions of polymeric plastics loaded with conductive solids such as carbon black, graphite, and finely divided metals have shown poor mechanical properties and are porous. In particular, thermoplastic polymers and conductive fillers were not deemed suitable for sophisticated applications such as a fuel cell. Nevertheless, in the invention, a bipolar plate was made from a mixture of crystalline propylene-ethylene thermoplastic copolymer and at least 30 parts by weight of carbon black and/or graphite per 100 parts by weight of copolymer. The mixture was prepared under high shear and at least 100xc2x0 C. to minimize degradation of the copolymer. The resulting product had a resistivity from about 0.5 to 10 ohms-cm. No fuel cell test data was shown. Resistivity levels shown may be too high for use of the plates in a fuel cell. Higher carbon loadings would therefore be needed, precluding the use of high shear mixing.
Another example of using graphite with a thermoplastic polymer for a bipolar plate is U.S. Pat. No. 4,339,322. The graphite and thermoplastic fluoropolymer were combined in a weight ratio from 2.5:1 to 16:1. Carbon fibers were added for strength and conductivity. The fibers were preferably hammer-milled fibers having an average diameter of 0.05 inches. The resulting bipolar plate was formed by compression molding and had a resistivity down to about 1.9xc3x9710xe2x88x923 ohms/inch. Although mechanical strength may be improved by this approach, the use of compression molding is likely to leave porosity at the interfaces between the fibers and the polymer matrix.
In U.S. Pat. No. 4,098,967, a bipolar plate for a lead-acid battery was made from a plastic filled with a glassy or vitreous carbon at 40 to 80% carbon by volume. The plastics included thermoplastics and fluorocarbon plastics. However, it was noted that the conductivity of vitreous carbon was substantially less than that of carbon black or graphite and, therefore, heavy loading was required. The resulting product has a specific resistance of about 0.0002 ohm-cm. The use of compression molding and high carbon loadings in this design is likely to provide poor mechanical properties and relatively high porosity.
In the context of another lead-acid battery, U.S. Pat. No. 5,141,828 discloses a mixture of thermoplastic polymer (such as polyethylene) and a uniform dispersion of carbon black for the bipolar plates. The carbon black was present at about 20 to 40% by weight. The polymer and carbon black were mixed in a solvent, the solvent was then evaporated, and the resulting mixture was pulverized to form a powder. The powder was then compression molded into plates. While it was claimed that the plates provided low internal resistance, no quantitative specifics were disclosed. The relatively low carbon content would provide better mechanical properties on compression molding, but the resistivity levels are likely to be too high for fuel cell operation.
A graphite powder bipolar plate for a zinc bromide battery was disclosed in U.S. Pat. No. 4,758,473. Prior vitreous carbon bipolar plates were noted to be brittle, expensive, and chemically unstable. It was further noted that prior bipolar plates made of graphite and thermoplastic fluoropolymers, as well as glassy carbon and plastic, did not provide low cost, durability, and good electrical performance. Thus, in the invention, graphite was heat treated up to about 800xc2x0 C. The heat-treated graphite was then mixed with a thermoplastic resin in a weight ratio between 1:5 and 1:1. The mixture was then pressure molded at 190xc2x0 C. No electrical data were provided, and it is unlikely that annealing at 800xc2x0 C. is high enough to improve the electrical conductivity of the graphite used.
Another zinc bromide battery used carbon black with a polymeric matrix (e.g., a polyproylene copolymer) for its bipolar plates in U.S. Pat. No. 5,173,362. The carbon black was present at about 5 to 40 wt. %. xe2x80x9cLong or continuous fibersxe2x80x9d (such as glass fiber mat) were also added in the mixture to provide reinforcement. The bipolar plate resistivity appeared to approximate 1.23 ohms/cm, which is likely to be too high for PEM fuel cell operation. Moreover, the process described may be difficult to scale-up.
U.S. Pat. No. 5,863,671 shows a bipolar plate that is constructed by a lamination of a core of non-conductive plastic or ceramic that is sandwiched between conductive platelets of metal or conductive ceramic or conductive plastic. The metal is typically titanium protected by a titanium nitride coating. No specifics of the conductive plastic component were given. The coated titanium used in one version was likely to have issues of corrosion and cost.
A past alternative to the use of graphite and carbon black is noble metals, such as platinum and gold. They have exhibited the needed qualities in bipolar plates. But they are prohibitively expensive to use.
As can be seen, there is a need for an improved electrically conductive plate bipolar plate while maintaining mechanical properties, and method of making the same in thin parts, for a viable PEM fuel cell. A bipolar plate is needed that exhibits adequate conductivity, is corrosion resistant, and is low in manufacturing cost. A bipolar plate is needed that also has good mechanical strength and is impermeable to hydrogen and air in fuel cell operating conditions.
In one aspect of the present invention, an electrically conductive flow-field plate in a proton exchange membrane fuel cell comprises a composition made of a resin and a plurality of carbon nanotubular fibers having an average diameter that is at least about 1 nm and up to about 300 nm. In another aspect of the present invention, an electrically conductive plate with molded channels for use in a proton exchange membrane fuel cell comprises a composition made of a resin and a plurality of carbon nanotubular fibers present at not more than about 85 wt. %.
In still another aspect of the present invention, a method of making a conductive plate for a proton exchange membrane fuel cell comprises compounding a mixture of carbon nanotubular fibers and a resin, with the carbon fibers being present at about 7 to 85 wt. %; and molding the mixture. Preferably, the carbon fibers are present at about 10 to 50 wt. % and the mixture is injection molded to obtain flow-field bipolar plates in near-net shape.